1. Field of the Invention
This invention relates to pressure swing adsorption systems for air separation. More particularly, it relates to the use of a single bed pressure swing adsorption system for the recovery of oxygen from air.
2. Description of the Prior Art
The pressure swing adsorption (PSA) process and system provides a commercially attractive approach for separating and purifying at least one component of a feed gas mixture containing at least one less readily adsorbable component and at least one more readily adsorbable component. Adsorption occurs in an adsorbent bed at an upper adsorption pressure, with the more readily adsorbable component thereafter being desorbed from the adsorbent bed by reducing the adsorbent bed pressure to a lower desorption pressure. The carrying out of the adsorption/desorption PSA process is well known and is disclosed, for example, in the Wagner patent, U.S. Pat. No. 3,430,418, relating to PSA systems having four or more beds. As disclosed in this patent, the PSA process is commonly carried out, on a cyclic basis, in a processing sequence that includes, in each bed, (1) pressure adsorption, with feed gas being introduced to the feed end of the bed and with release of the less readily adsorbable component, as product gas, from the product end of the bed; (2) cocurrent depressurization to intermediate pressure with release of void space gas from the product end of the bed; (3) countercurrent depressurization to a lower desorption pressure, with release of the more readily adsorbable component from the feed end of the bed, (4) optional purge at the lower desorption pressure, with purge gas being passed to the product end of the bed to enhance removal of the more readily adsorbable component, desorbed from the adsorbent bed, from the feed end of the bed; and (5) repressurization from the lower desorption pressure to the upper adsorption pressure, so that the cycle can be repeated with additional quantities of feed gas being passed to the bed. The void gas released during the cocurrent depressurization step is commonly employed for pressure equalization purposes between beds in the multi-bed system and to provide purge gas to a bed at its lower desorption pressure. Variations of such processing sequence are employed in the art for use in systems containing one or more adsorbent beds.
In numerous chemical processing refinery, metal production and other industrial applications, high purity gas streams are frequently employed for a variety of purposes. For example, high purity oxygen is used in various industries, such as chemical processing, steel mills, paper mills and in lead and glass production operations. Many such applications typically require oxygen purities in the 90-93% range, with flow rates of up to 100,000 ft..sup.3 /hr or more. PSA processing is well suited for air separation to produce oxygen, and nitrogen, by various processing techniques such as indicated above, and variations thereof, particularly in operations of a relatively small size for which the use of a cryogenic air separation plant may not be commercially feasible. In such PSA systems designed for the recovery of high purity oxygen product as the less readily adsorbable component of air, such adsorbent bed will commonly contain an adsorbent material capable of selectively adsorbing nitrogen as the more readily adsorbable component of air, with said nitrogen being subsequently desorbed and removed from the bed upon reduction of bed pressure to the lower desorption pressure, as the cyclic adsorption/desorption sequence is carried out in the PSA system.
In PSA-oxygen and other PSA processing applications, it is desirable to minimize design, fabrication, transportation and equipment costs in order to achieve lower capital costs and increased system reliability. Accordingly, it is desirable to use PSA systems and processes that minimize the number of operating components, such as adsorbent vessels, adsorbent inventory, related tanks, valves, compression equipment, process lines and the like. The costs associated with the operation of PSA systems are minimized by the use of processes that require lower power per unit of product produced. Such process for oxygen production desirably exhibit high recovery of oxygen from feed air, while enabling low compression ratios to be employed in the associated compression equipment.
PSA systems containing four or more adsorbent beds, as in the Wagner patent referred to above, are desirable for certain gas separation operations, particularly to achieve high volume, high purity and high recovery performance. In other applications, it is possible and desirable to employ two or three bed PSA systems. For example, it has been found desirable to employ two bed PSA systems for various practical commercial PSA-oxygen applications. The inlet gas mixture to be separated is normally compressed to a higher pressure, usually several atmospheres, before the desired selective adsorption occurs. The resulting high pressure product gas e.g., the less selective oxygen component of feed air, is passed in a pipeline for downstream use. In typical 2-bed PSA operations, transatmospheric pressure conditions are employed. Thus, part of the cycle is run at pressures above atmospheric, and part at pressures below atmospheric pressure. The separation generally takes place at pressures very close to 1 atmosphere, and the product gas is then compressed to the desired downstream pressure. This has been found to be a cost-saving manner of operation, since the necessary gas compression is essentially that for product flow, not for the entire inlet feed gas stream. However, since the PSA processing cycle involves both pressure and vacuum conditions, two machines, i.e., a feed blower and a vacuum blower, are needed to implement the cycle. Such a 2-bed system is cost effective for oxygen plant capacities in the size range of 20,000 NCFH to 50,000 NCFH or more of oxygen. For applications with flow requirements that fall below this range, the capital costs associated with a two-bed vacuum pressure swing adsorption (VPSA) system make such processing uneconomical.
Single bed, single machine embodiments of the VPSA system have been proposed in the art to lower initial capital costs so as to render the VPSA system more suitable for lower flow rate applications. Since a large portion of the capital costs associated with a VPSA-oxygen plant relates to the cost of air blowers, the processing vessels, and the adsorbent for use in said vessels, it will be appreciated that the capital costs can be significantly reduced by cutting in half the number of blowers and the number of adsorbent vessels included in the system, whether for trans-atmospheric operations or systems in which vacuum conditions are not employed. In addition, a single bed PSA system employs much fewer valves than are needed to operate multi-bed PSA systems.
A single bed PSA system has been described by McCombs et al. in U.S. Pat. No. 4,561,865. In the McCombs et al. approach, a pressure controlled valve in the discharge line from the top of the adsorbent bed opens when the pressure at the top of the single processing vessel is at a certain level. Gas is passed in the discharge line directly to an external equalization tank. In the embodiment of the McCombs et al., there is also a by-pass line off the discharge line on the outlet of such pressure control valve, containing a check valve, said line passing to a product surge tank. When the pressure in the product surge tank is equal to that in the equalization tank, the check valve opens, and product gas is fed into the equalization tank and the product surge tank simultaneously. The check valve precludes the back flow from the product surge tank into the adsorbent bed when the bed pressure falls below that of the product surge tank. In this arrangement, the equalization tank serves as an extension of the product surge tank, since both tanks pressurize to the same top pressure, and contain the same purity gas. McCombs et al. also teach the use of a blowdown step to partially depressurize the adsorption vessel prior to evacuation.
While the McCombs et al. patent addresses the need in the art for a single bed PSA system, further improvement is needed in order to enable such single bed operation to satisfy the needs of the art with respect to low flow rate applications. More particularly, a higher product flow rate is desired in the art as compared to that obtainable by the McCombs et al. approach and other such single bed PSA systems.
It is an object of the invention therefore, to provide an improved, low volume PSA process and apparatus for the recovery of oxygen from air for low volume applications.
It is another object of the invention to provide an improved single bed PSA process and apparatus for the low volume recovery of oxygen from air.
It is a further object of the invention to provide a single bed, low volume PSA process and system for the enhanced flow of product oxygen recovered from air.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.